1. Field
Exemplary embodiments of the present invention relate to a semiconductor design technology and, more particularly, to a semiconductor memory device for performing a refresh operation.
2. Description of the Related Art
A semiconductor memory device includes a plurality of memory banks for storing data. Each of the plurality of memory banks may include tens of millions of memory cells or more. Each of the memory cells includes a cell capacitor and a cell transistor. A semiconductor memory device stores data through an operation for charging or discharging electric charges in or from a cell capacitor. The amount of charges stored in the cell capacitor ideally needs to remain constant unless the cell capacitor performs any specific operations. However, the amount of charges stored in the cell capacitor may vary due to a voltage difference with peripheral circuits. Electric charges may be drained from a cell capacitor that has been charged or electric charges may be introduced into a cell capacitor that has been discharged. As described above, a change in the amount of charges of the cell capacitor means a change of data stored in the cell capacitor. This means that the stored data is lost,
A semiconductor memory device performs a refresh operation to prevent such a loss of data.
As the process technology is advanced, the degree of integration of semiconductor memory devices is gradually increased. An increase in the degree of integration of semiconductor memory devices also influences the size of memory banks. A gradual reduction in the size of memory banks means a reduction of the interval between memory cells. This means that the interval between word lines respectively coupled to adjacent memory cells is reduced. As the interval between adjacent word lines is reduced, new issues are raised. For example, a coupling effect is generated between adjacent word lines. If such a coupling effect is generated between adjacent word lines, memory cells coupled to the adjacent word lines may not retain data stored therein. That is, a risk of losing data is increased.
To prevent such issues, a semiconductor memory device has only to perform a refresh operation on all the memory cells of memory banks. That is, the number of refresh operations may be increased to the extent that a loss of data can be prevented. However, if the number of refresh operations is increased, operation efficiency of a semiconductor memory device is deteriorated.